Preparation and Studies of Adsorption Properties of Microporous Carbon Spheres
Engineering and Protection of Environment
The modified Stöber method involving a one-step simultaneous calcination and activation was used to obtain highly microporous carbon spheres. Resorcinol and formaldehyde were used as carbon precursors, potassium oxalate was employed as an activating agent and carbon source, and ammonia was used as a polymerization catalyst. The resulting spherical phenolic resins containing potassium salt were subjected to simultaneous carbonization and activation at 600°C for 4 hours in flowing nitrogen.
... ing nitrogen. However, non-activated carbon spheres (without potassium oxalate) were carbonized at 600°C for 2 hours in flowing nitrogen. The simultaneous carbonization and activation of polymeric spheres afforded carbon spheres with much higher microporosity than that in the spheres obtained without potassium salt. In the case of activated spheres each the specific surface area, the total pore volume and the micropore volume increased about twice. The obtained carbon spheres featured the specific surface area of 1490 m 2 g -1 , total pore volume of 0.74 cm 3 g -1 , the ultramicropore volume of 0.38 cm 3 g -1 and the micropore volume of 0.61 cm 3 g -1 . A well-developed microporosity, in particular ultramicroporosity (pores of sizes below 1.0 nm), has an essential influence of adsorption of CO2. The activated spheres adsorbed 7.67 mmol g -1 at 0°C and 1 atm. These spheres featured also high working capacity with respect to CO2 equal 4.23 mmol g -1 estimated as the difference between the gas uptake at 30°C and 1 atm and the gas uptake at 60°C and 0.0013 atm. The pore size distributions calculated from nitrogen adsorption isotherms at -196°C and from CO2 adsorption isotherms at 0°C by using the density functional theory for heterogeneous surfaces, 2D-NLDFT, are also shown. These distributions confirmed a significant development of microporosity in the activated carbon spheres. Also, the isosteric heat of CO2 adsorption was calculated by using CO2 adsorption isotherms measured in the temperature range from 0 to 60°C. The isosteric heat of adsorption on activated carbon spheres varies from 40 kJ mol -1 to about 25 kJ mol -1 . This study shows that a simultaneous carbonization and activation of phenolic resin spheres affords carbon spheres with high microporosity suitable for CO2 capture at ambient conditions.